Since the Duggan test method page was
completed prior to this page, a considerable
amount of information regarding ettringite
in concrete has already been written there.
You may
want to visit that page before continuing
here.

The Duggan test method can be used to predict
the potential for the generation of excessive
ettringite in a portland cement concrete
(PCC) mix design.

Delayed ettringite formation (DEF) is
sometimes called an internal sulfate attack.
An external source of sulfur is not required
for this type of early PCC deterioration to
occur.

The type of ettringite formed can appear to
be crystalline, but when analyzed by
qualitative x-ray diffraction (QXRD), it is
amorphous. In some cases, crystalline
ettringite can also form. Extreme care must
be exercised when quantifying and qualifying
ettringite in concrete when using the x-ray
diffractometer. Some purists claim that if it
can not be identified by QXRD, then it is not
ettringite, which is technically true. But
what if this ettringite is a gel (or
proto-crystalline), similar to the
relationship of silica gel to quartz?

Element mapping with the scanning electron
microscope (SEM) shows this material
(ettringite gel?) to be mainly a calcium
sulfoaluminate. True crystalline ettringite
has approximately 32 water molecules tied to each calcium sulfoaluminate
molecule. What the water arrangement in an ettringite
gel would be is unknown, but researchers in
California have worked on that problem. The
SEM can not be used to quantify water in the
ettringite gel because hydrogen is too light
to detect. Thermogravimetric analysis (TGA)
should be able to solve this problem.

This fiberous, opaque, amorphous ettringite
can be easily seen in micro-cracks and in air
voids of modern PCC. It is especially
prominent in some PCC pavements suffering
from very early ( 4 years in some cases)
deterioration.

At Iowa State University (ISU) in Ames, Iowa,
the x-ray diffractometer was used in an
attempt to identify this material (ettringite
gel) that was accumulating in the air voids
of early deteriorated portland cement
concrete pavements (PCCP). The ettringite
gel material (fibous and like a thick paste)
was
removed from the air voids and put on a
quartz sample holder. A one hour run,
concentrating on the zone where the ettingite
peaks should occur, showed only a halo
associated with amorphous material.

Later, a PCCP researcher from Minnesota used
the SEM and diffractometer to analyze 20 year
old, severly deteriorated PCC. From the
fractured surface of the concrete, hard balls
of ettringite were removed. These hard balls
of ettringite were from air voids over 100
microns in diameter. Some of the ettringite
balls were clear and some were opaque. The
clear ettringite balls and the opaque balls
were pulverized and analyzed by XRD. Both
tests showed the material to be amorphous.
The ettringite balls, after being pried out
of their air void sockets, had the appearance
of the surface of a human brain.

I is my opinion that the ettringite fibers,
found in air voids, are growing at the base
rather than at the top and that individual
fibers come together to form bundles. When
looking at air voids completely full of
ettringite, it appears that a squashing from
the outside edge has taken place. Also, when
examining (with the SEM) laboratory concrete,
containing 15%
fly ash, a few weeks after fabrication, fly
ash spheres, lifted off the surface of the
air voids, could be seen sitting on top of
ettringite bundles. Initially, it was thought
that ettringite might be extruding from pores
in the air void surface. Another explanation
would be that the ettringite is forming at
the
surface and pushing the ettringite fiber up.
This explanation is not as absurd as it first
sounds. Fresh water ice domes occur in salty
lakes in the upper Andes. Fresh water seeps
upwards through the base material of the
shallow, salt water lakes, where it then
combines with bulk ice, forcing the ice
islands to rise above the surface of the cold
(below freezing temperature) salt water.

PCC researchers in Demark have analyzed this
PCCP
ettringite material for a
longer period of time on XRD equipment and
have said that they were able to identify the
ettringite peaks. It was not disclosed if
these were small peaks on top of an amorphous
halo.

Researchers from England,
Schlumberger Research
have published reports that indicate
phosphonate concrete retarders can "poison"
ettringite gel and hinder its developement
into crystalline ettringite. Are other
dispersent/surfactant concrete additives able
to do likewise?

After observing this amorphous ettringite
over time, identification can usually be made
from its physical appearance. On occasion, a
thin coating of silica gel can cover
ettringite and cause some confusion when
doing element mapping with the SEM. An
element map of the cross section will usually
solve the problem as well as determine the
sequence of events.

Some PCCP petrographers, using optical
equipment, are apparently confusing
ettringite gel for silica gel. The
SEM can readily separate ettringite gel from
silica gel, contrary to the opinion of some
petrographers. In fact, the SEM will quickly
and easily identify the alkalis in the silica
gel. In addition to sodium and potassium,
calcium was seen quite often as the major
metal in silica gel.

Many samples of deteriorated PCCP, from other
states, were analyzed with the equipment at
ISU. Looking at modern, early deteriorated
PCCP from southern states was particularly
helpful in determining the role of
freeze/thaw in Iowa's early deteriorating
PCCP.

PCCP from I-20 east and west of Monroe, La
was
analyzed with ISU's equipment. I-20 began to
deteriorate in less than 5 years. Both
sections east and west of Monroe deteriorated
equally, but only one section contained fly
ash. The coarse-aggregate in the concrete
contained some argillaceous (shaley)
particles. These particles would not perform
well in Iowa PCCP (freeze/thaw cycles &
deicing salts) but apparently can perform
well in Louisiana PCCP as service records
attest. Images and element maps from the SEM
showed a considerable amount of ettringite in
the PCCP void system. Other petrographers,
who analyzed this PCCP, said there was little
or no ettringite observable even though a
single SEM element map, published in their
report, showed air voids full of ettringite.
A water reducer/retarder was used on the
projects.

PCCP from Wisconsin was also analyzed. The
type of deterioration was quite different
from the other early deteriorated PCCP. The
early deterioration in the Wisconsin pavement
took the form of a 5 inch deep "V" trough at
the sawed joint. A mostly igneous gravel
(good service record) was
used for coarse-aggregate in the PCCP mix.
The cement was relatively high in the amount
of sulfur and potassium. SEM analysis showed
the air voids
to be full of ettingite. Wisconsin DOT
engineers said other roads, built at the same
time with the same design, but with the use
of a carbonate coarse-aggregate, were
performing adequately. While it is possible
to have early PCCP deterioration problems
caused by delayed ettringite formation (DEF),
without the application of external heat, the
application of heat can take a marginal
situation over the edge particularly if the
concrete matrix looses its integrity. The
application of
heat, in the Duggan test method pretreatment
process, probably produces micro-cracks, in
the matrix,
that allow water to get to unreacted
ettringite. Could the sawing of PCCP, made
with igneous coarse-aggregates, and a mix
susceptible to the generation of excessive
ettringite relate to the cause of this very
selective early PCCP deterioration? Anyone
who has ever sawed concrete, made with
igneous aggregates, has seen red hot
particles. Materials in the sand fraction
can also glow red-hot when sawed.

Some concrete investigators say that a
general expansion of the matrix occurs when
PCCP fails due to DEF. In-house studies at
the Iowa DOT would support that conclusion.
However, when observing aggregate particles
in a polished PCC surface, ettringite does
not have to completely surround the
particles.
Quite often the aggregate particles will
remain in contact with part of their sockets.
The amount of ettringite, if any, will depend
on where the slice bisects the
aggregate/paste.

PCC box beams from Texas were also studied
using the equipment at ISU. These beams were
made with type III cements from two sources.
The coarse-aggregate was crushed
limestone containing a minor amount of porous
chert. The coarse aggregate had been used
previously and had an acceptable service
record. SEM images showed expansion taking
place in a preferential (transverse)
direction. Micro-cracks. filled with
ettringite, were parallel to the edges of the
beam. Porous chert becomes expansive when
used in PCC, however, good PCC can tolerate
some porous chert with no ill effects. The
interior tensile strength of the matrix will
exceed the expansive energy of the reacive
chert, particuarly if the pore system in the
matrix can handle the silica gel. The
situation near the surface is different and
popouts can occur. When general expansion
of the matrix (due to DEF) occured in these
box beams, some of the micro-cracks cut
across some reactive chert particles. In
these
cases, the matrix and aggregate particle were
both expansive. SEM element mapping of these
areas showed silica gel in the reactive chert
crack (just to its border) and ettringite in
the matrix crack (just to its border with the
chert particle. Neither the silica gel or the
ettringite intruded into each others space,
indicating nearly equal expansion energy.

The type III cement used in the fabrication
of these Texas beams, like most other type
III cements, contains more sulfur than type I
cements. Type III cements are made to a
finer grind to attain high early strengths
when used in PCC. Most likely, fine-grind
cements would have a greater problem with
flowability (out of the silos) than coarser
grinds. A potential problem (DEF in concrete)
could occur if the extra sulfur in the type
III cement is tied to potassium as arcanite.
The amount of arcanite (K2SO4) in cement
relates directly to the amount of syngenite
generated in cement silos. How much (and how
aggressive) grinding aid is needed to control
flowability under these conditions? The use
of a type III cement, containing a
significant amount of grinding aid, in a mix
containing a super-plasticizer could lead to
DEF related problems. An Iowa PCC pavement,
made with type III cement, failed
prematurily.

The states of Ohio and New York have used
type K (shrinkage compensating or expansive)
cement in pavements and structures. Ohio's
experience has been positive. New York has
experienced some minor failures with
structures. Filling the concrete pore system
with ettringite while maintaining matrix
integrity could be beneficial for initial
strength and stopping chloride penetration.
How do we guarantee long-term matrix
integrity? Can we guarantee that all of the
ettringite will form during the plastic stage
of hydration or that water will never reach
any
unreacted ettringite in the future? Would
remedial techniques, such as furnishing
abundant reservoirs (air voids) for DEF,
solve the problem?
Ohio PCC, made with type
K cement, was analyzed with the SEM. Image
analysis showed the sample to contain nearly
10% air voids. Type K cement works for Ohio
PCC.

ASTM specifications allow for the additional
sulfur in type III cement because of the
additional surface area of the C3A particles
due to fine grinding. Gypsum, hemi-hydrate
(bassanite) and/or anhydrite (calcium
sulfates, some hydrous) are introduced
into the operation during clinker grinding.
The potassium sulfate (when present) is
usually derived from
the cement clinker.